pre-RFC: let build scripts talk to cargo via a native Rust API instead of string printing

[Jules-Bertholet]

@kornel

know this currently can be provided by a crate (and the API should be prototyped and ironed out as a crate)

Futher extensions to `cfg(target_family)`, and concerns about breakage

Even more interesting... https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts CARGO_CFG_TARGET_FAMILY=unix — The target family, either unix or windows . The one place you can check what the cargo buildscript environment variables are specifically says that only unix or windows are allowed values. This isn't true, and clicking through to the #[cfg] docs makes this clear, but the env var docs are wrong here. In general, the …